A leading Kosovo Serb politician was shot dead outside his office and Serbia says it's an act of terrorism

Police officers guard a scene where unknown assailants opened fire on Kosovo Serb politician Oliver Ivanovic in front of his office in the northern, Serb-dominated part of Mitrovica, Kosovo, January, 16, 2018. Associated Press

Ethnic Serb politician Oliver Ivanovic led a small political party in Kosovo and was known as a relative moderate.

Though he had a troubled past for previous actions against Kosovo's Albanian majority, he was known for cooperating with NATO, UN and EU officials in an order to establish a strong democracy in Kosovo.

Tensions between Kosovo's Albanian majority and Serb minority are becoming increasingly strained, especially in the divided town of Mitrovica, where Ivanovic lived.

PRISTINA/BELGRADE (Reuters) - Kosovo Serb leader Oliver Ivanovic, who was standing trial over the killings of ethnic Albanians during the 1998-99 war, was shot dead on Tuesday in what Serbia called an act of terrorism.

The killing increased tension between Kosovo and Serbia and an ongoing round of European Union-sponsored dialogue on a normalization of relations that was due to take place in Brussels was suspended at the request of Belgrade.

The European Union and United States called on both sides to remain calm and for Kosovo authorities to bring the killers to justice without delay. They also urged authorities in Kosovo, a former Serbian province that won independence a decade ago, and Serbia to recommit to working for a normalization of relations.

Serbian President Aleksandar Vucic called an emergency session of the country's National Security Council.

"For the Serbian state this is considered an act of terrorism," Vucic said afterward, and Belgrade would demand to be included in the investigation. He declined to comment on whether he believed the killing was ethnically motivated.

Ivanovic, 64, was gunned down in front of his party office in Mitrovica, a town bitterly divided between ethnic Serbs and Albanians, shortly after 8 a.m. (0700 GMT). He was taken to a hospital but doctors failed to revive him.

There were no witnesses to the attack and no shots were heard, suggesting the weapon used had been fitted with a silencer, Serbian media reported. Tanjug news agency quoted his lawyer as saying Ivanovic was shot at least five times.

Police said they found a burnt-out Opel car in the town after the shooting and suspected it was linked to the attack. They offered 10,000 euros ($12,225) to anyone with information that would resolve the case and provided a secure phone line.

The Kosovo government, led by some former commanders of ethnic Albanian guerrillas who rose up against Belgrade's repressive in the late 1990s, also condemned the attack.

"The killing of Oliver Ivanovic challenges the law and any attempt to establish law and order throughout the entire territory of Kosovo," it said in a statement.

Marko Djuric, director of Serbia's government office for Kosovo, said Ivanovic's killing was aimed against the Serbian "people that must and will be punished."

"Bridge-watchers"

Known as a relative moderate among mainly ultra-nationalist Kosovo Serb politicians, Ivanovic became one of the chief interlocutors for NATO, U.N. and EU officials based in Kosovo to help steer it on a firm path to democracy and the rule of law.

In 2016, he was convicted of war crimes linked to the killings of four ethnic Albanians during the 1998-99 war and was jailed for nine years. But after a retrial was ordered last year, he was released and allowed to defend himself while free.

Serbian officials dismissed the accusations against Ivanovic and said the process against him was staged.

Ivanovic came to prominence shortly after the war as one of the community leaders of Mitrovica Serbs known as the "bridge-watchers" who sought to prevent "infiltration" by Albanians over the Ibar River bridge into the northern half of the town.

The bridge-watchers were often involved in ethnic violence in the early post-war period, after NATO air strikes drove out Belgrade forces accused of killing and expelling Kosovo Albanian civilians in a counter-insurgency campaign.

The group later disbanded but many members branched out into organized crime. Ivanovic entered politics and became known for a pragmatist stance advocating dialogue and compromise with Kosovo Albanians, though still rejecting Kosovo sovereignty.

Some 40,000-50,000 ethnic Serbs live in northern Kosovo, rejecting integration with the mainly ethnic Albanian state.

Relations between Serbia and Kosovo have been tense since 2008, but in 2013 both parties agreed to participate in EU-sponsored negotiations on normalizing relations, a condition for both to progress on their way toward membership in the bloc.

(Additional reporting by Aleksandar Vasovic in Belgrade and Gabriela Baczynska in Brussels; writing by Ivana Sekularac; editing by Mark Heinrich)